This invention relates to two-terminal non-linear devices and methods for their fabrication.
In certain electronic applications there exists the need for two-terminal non-linear devices whose physical dimensions are very small, and much smaller than the dimensions of commonly available discrete non-linear devices. One such application is active matrix switching in which two-terminal non-linear devices function as control switches. An example of this application is active matrix addressed electro-optical display devices having a row and column array of individual display elements, such as liquid crystal display elements, and in which one or more two-terminal non-linear device in the form of diode structure is connected in series between a display element electrode and an associated address conductor. These devices are located adjacent their respective display elements. For TV picture display purposes the display elements are generally rectangular in shape and usually have a maximum dimension of around 200 .mu.m. The physical size of the associated non-linear device needs to be much less so that as great a proportion as possible of the area occupied by a display element electrode and its associated non-linear device is devoted to the display element electrode for maximising display effect. Typically, such a display device can have around 200,000 display elements.
Because of the large numbers required and the small size demanded for the non-linear devices in such applications, the conventional approach hitherto has been to fabricate the non-linear devices in situ as individual multi-layer thin film diode structures in an array on a common substrate, together with the display element electrodes and address conductors, using complex photolithographic techniques. Non-linear devices in the form, for example, of MIM, back to back, or n-i-n diode structures have been employed, typically occupying a maximum, lateral, dimension of a few tens of microns, say 30 .mu.m. Although this approach has been generally accepted, the extensive photolithography required in depositing and patterning the series of thin film layers entailed has certain drawbacks, most particularly because of the desire for increasingly larger display devices and the complexity and expense of the equipment necessary to achieve this.
In the Applicant's co-pending British Patent Application No. 8920963.9 (PHB33587) there is described an alternative approach to constructing active matrix arrays which is especially suited to producing large area display devices. This involves providing the address conductors and display element electrodes in planar arrays spaced from one another with prefabricated, discrete, two-terminal non-linear devices situated between the two arrays and electrically connected between the display element electrodes and the address conductors. The discrete devices can be distributed over one or other of the arrays in a quasi-random manner before bringing the two arrays together or alternatively distributed in quasi-random manner in a polymer film which is disposed between the two arrays. The density of the distributed devices is such as to ensure that at least one device is situated between each display element electrode and the associated conductor. For more details of this active matrix display device reference is invited to the aforementioned application, whose disclosure in this respect is incorporated herein by reference.